Scintillator ceramics or bodies are used for identifying high-energy radiation such as X-rays, gamma rays and electron beams. These bodies contain a luminescent material which absorbs the high-energy radiation and converts it into visible light. The luminescent radiation produced thereby is electronically detected and evaluated using light-sensitive systems such as photodiodes or photomultipliers.
For highly sensitive radiation detectors such as are required, for example, in X-ray computer tomography, scintillator ceramics made from pigment powders of the rare earth oxysulfides are known which obey the generic formula(M1-xLnx)2O2S
These are therefore metal oxysulfides (MOS) which have been doped with specific rare earth elements (Ln). In the generic formula, M stands for Y, La and/or Gd and Ln for an element from the group Eu, Ce, Pr, Tb, Yb, Dy, Sm and/or Ho.
For a high light yield when converting the high-energy radiation, the scintillator ceramic must be optically translucent to transparent in order to ensure high transmittance of the luminescent radiation. A high quantum efficiency is additionally required for the conversion. An excessively high afterglow is undesirable.
A high degree of transparency of the scintillator body can only be achieved with a high-density ceramic having an extremely low residual porosity. Optimum transmission of the luminescent radiation is adversely affected not only by crystal anisotropy of the optical refractive index but also by secondary phase inclusions as well as grain boundaries and, in particular, voids.
To produce an optimum scintillator ceramic, a metal oxysulfide powder of the appropriate (required) composition must be transformed into a compacted powder body which is then densified by sintering at elevated temperatures to form a ceramic that is as void-free as possible. As the metal oxysulfides chemically decompose at high temperatures, the sintering result cannot be sufficiently optimized by simply increasing the sintering temperature. This disadvantage can be largely compensated by applying mechanical pressure during the sintering process.
Corresponding pressure sintering technologies are used for producing high-transparency scintillator ceramics from metal oxysulfides. DE 36 29 180 A1 and DE 37 02 357 C2 describe the production of scintillator bodies wherein hot isostatic pressing is used during the sintering process. The operations involved here are very complex and the procurement costs for the apparatus required are also relatively high.
By considerably increasing the specific surface area of the starting powder from <1 m2/g to >10 m2/g, it has been possible to increase the sintering activity in the powder body to the point that expensive hot isostatic pressing has been able to be replaced by less expensive uniaxial pressing. The individual process conditions involved are described in DE 42 24 931 C2. Because of the way in which the metal oxysulfide powders used are produced, the particles of these more sinter active powders consist of a large number of primary particles which form stable, hard and porous agglomerates or aggregates with particle sizes of between 30 and 85 μm. This results primarily in relatively low and inhomogeneous bulk densities, i.e. density gradients occur in the powder body so that pressure-assisted sintering is required.
A disadvantage common to sintering assisted by hot isostatic pressing and sintering assisted by hot uniaxial pressing is that for economic reasons it is only possible to produce comparatively large ceramic blocks which then have to be split up into smaller components by means of expensive and time-consuming cutting and sawing operations. This can result in considerable material losses of up to 50% of the original material.
The existing manufacturing processes for scintillator ceramics therefore require high capital investment for the machinery required and involve considerable process costs.